1. Field of the Invention
The present invention relates to handoff in a wireless communication system. More particularly, the present invention relates to a technique for Macrocell to Femtocell and Femtocell to Femtocell handoff in a wireless communication system.
2. Description of the Related Art
In a typical wireless cellular communication system, service, such as a voice and/or data service, is provided to Mobile Stations (MSs) through a plurality of Macrocell Base Stations (MBSs). Each of the MBSs is responsible for providing service to MSs located within their respective service coverage area. The service coverage area of an MBS is hereafter referred to as a Macrocell. To facilitate mobility of the MSs, handoff between MBSs is performed when the MS leaves one Macrocell for another.
In the wireless cellular communication system, a channel may deteriorate due to a number of factors, including a geographical factor inside a Macrocell, a distance between an MS and an MBS, movement of the MS, etc. Channel deterioration is problematic since it may result in a disruption of communication between the MS and the MBS. For example, when the MS is located inside a structure, such as an office building or a house, a channel between the MBS and the MS may deteriorate due a shadow region that is formed by the structure. A shadow region formed within the structure is hereafter referred to as an indoor shadow region. The MS located in the indoor shadow region may not be able to adequately perform communication with the MBS.
Recently, a Femtocell concept is proposed in order to provide ubiquitous connectivity to MSs while addressing a service limitation of an indoor shadow region. The term “Femto” is a prefix denoting a factor of 10−15 in the International System of Units. In the context of telecommunications, the term “Femtocell” refers to the service coverage area of a Femtocell BS (FBS) and is much smaller than a Macrocell. And, the term “FBS” refers to a low cost small cellular BS that is designed to service a small number of MSs as compared to an MBS. In addition, installation of the FBS is significantly easier than installation of the MBS and may be performed by a user of an MS.
The FBS enables service providers to extend service coverage of the wireless cellular communication system indoors, especially where access would otherwise be limited or unavailable, such as the indoor shadow region. In order to provide the service of the wireless cellular communication system, the FBS may communicate with the backbone network of wireless cellular communication system through a commercial broadband network.
The deployment of an FBS improves both the coverage and capacity of the wireless cellular communication system. Since the FBS allows a small number of MSs to use dedicated air resources in its reduced size coverage area, unlike the MBS in which bandwidth is shared by a large number of users, it is possible to provide a high level of service. Going forward, the advantages of deployment of the FBS are expected to be increasingly leveraged in wireless cellular communication systems.
In the meantime, similar to the need for handoff between MBSs, handoff between the Femtocell and the Macrocell, and handoff between Femtocells, is a key function in securing service continuity of an MS.
Since a large number of Femtocells can be deployed within a Macrocell, there are many problems to be addressed to enable an MS to perform handoff between the Femtocells and Macrocells, especially handoff from the Macrocell to a Femtocell, and handoff between the Femtocells.
One technique to perform handoff between the Femtocells and Macrocells, and between the Femtocells, would be to use the technique employed for handoff between the Macrocells. To facilitate handoff between the Macrocells, each MBS is populated with information on neighboring MBSs. Since there are a finite number of neighboring MBSs, the neighboring MBSs may be manually identified during cell planning and used to populate a neighbor list that is stored at each MBS. Also, in a given area, Macrocell Pseudo-random Noise (PN) codes are unique to each MBS and do not overlap. This allows mapping of a neighboring Macrocell PN code to its corresponding Cell Identifier, which enables handoff routing.
However, the technique described above for populating the neighbor lists with information on neighboring MBSs does not scale for Macrocell to Femtocell handoffs for two reasons. First, the number of Femtocells within a Macrocell could be so large that populating individual Femtocell identifiers is not feasible. Also, Femtocells typically re-use Pilot PN codes, so mapping of a Femtocell PN code to Femtocell Cell Identifier at an MBS may not be possible.
Therefore, a need exists for a technique for Macrocell to Femtocell and Femtocell to Femtocell handoff in a wireless communication system.